The present invention relates to an apparatus and to a method for operating a tape storage device, and to a related computer program product.
In tape storage devices data typically is written to and read from a tape inserted into a tape drive of the tape storage device. The tape may be preformatted into data bands and servo bands extending next to each other in a motion direction of the tape. While the data bands provide data tracks for writing data to, the servo bands comprise prerecorded servo patterns, also denoted in a more general way as servo information, which servo information may be read by a reading element of the tape head for gaining information about a lateral position of the tape with respect to a reference position. Lateral in this context is meant orthogonal to a reference motion direction of the tape. A deviation of the lateral position from the reference position detected with the aid of the read element is also denoted as position error signal PES in this technical field. Such information, and specifically the PES, may be provided to a feedback controller, also denoted as lateral position feedback controller which converts the PES into a control signal for a realigning between the tape head and a data track currently read or written and specifically its track centerline for enabling a correct reading and writing of data. As a result, a closed-loop control system for a continuing alignment of the tape head and the tape is provided which is also denoted as closed-loop track-follow control system.
One of the key challenges for such a closed-loop track-follow control system originates from lateral vibrations of the tape. In traditional tape paths, flanged rollers are used for constraining a lateral tape motion during a transport of the tape from a supply reel to a take-up reel. Although the use of flanged rollers constrains the tape and limits lateral tape motion during operation, debris accumulation may be introduced at the flanges that not only may affect the lifetime of the tape, but may also create undesirable dynamic effects due to a contact of the tape edge with guiding elements such as flanges. These effects may be alleviated by removing the flanges from the rollers and operating the tape path without guiding elements.
However, such approach may introduce new challenges: Removing the guiding elements also removes a constraint on the lateral motion of the tape, which may result in an increase in amplitude of a typically low-frequency lateral tape motion. Specifically, an effect denoted as stack shift may become more prominent. Stack shifts are due to tape-stacking irregularities in the cartridge reel that lead to lateral tape motion events during normal operation of the drive. In general, stack shifts appear as high-amplitude low-frequency disturbances that can repeatedly be observed at the same longitudinal position when using the same cartridge.
An increased amplitude of a lateral tape motion may in addition increase a tape skew which in turn may degrade the performance of the tape storage device. Therefore, a compensation mechanism with respect to tape skew is desired.